Certain manufacturing processes call for coating thin films of materials onto various commercially important substrates. One method that has been used commercially for applying materials onto a substrate is spin processing or spin-coating, using a spin-coater. A spin-coater allows placement of a quantity of a material onto a substrate, and can rotate the substrate about its central axis through one or a series of rotational speeds. Centrifugal action causes the material to spread out over the surface of the spinning substrate, e.g., into a thin, uniform film.
More generally, processing of various commercially important substrates, e.g., microelectronic devices such as semiconductor wafers and integrated circuits, requires that some process steps be limited to well-defined areas of the surface of a substrate. This is true, for example, in processing microelectronic devices, to precisely place different materials onto a semiconductor wafer to construct circuit designs. A step of such a process is to precisely delimit the different areas of the substrate that must be either processed or protected from the actions of non-applicable materials and processing steps. A common method of processing such substrates is to use photolithography and spin-coating.
Photolithography is used to selectively protect or expose areas of a substrate such as a microelectronic device. A coating of a photosensitive photoresist material is spin-coated as a thin layer onto the device. The photoresist layer is exposed to electromagnetic energy through a patterned photomask, causing a chemical reaction within the exposed photoresist material, but not within the materials of the masked area (i.e., not exposed to electromagnetic energy). Afterwards, a developer solution is applied or spin-coated onto the entire photoresist material. The developer solution causes either the exposed or unexposed areas of the photoresist to be “developed” and allows removal of the developed or undeveloped photoresist. If the photoresist is of a so-called negative type, the unexposed area of the coating can be developed and removed; if the photoresist is of a so-called positive type, the exposed regions of the photoresist coating can be developed and removed. In both types of photolithography, the remaining photoresist forms a protective layer in either a positive or a negative pattern of the photomask that allows further processing of the exposed areas while protecting the areas covered by the photoresist.
The thickness of the photoresist layer (just prior to exposure) can have significant effects on one or more of the quality, performance, and cost of manufacture of the end product microelectronic device. The thickness of the exposed and developed photoresist layer can affect the size and resolution of features that can be constructed on the substrate using the photoresist layer. A thinner layer will allow finer features and finer resolution of features, based on a range of useful aspect ratios (i.e., height versus width) of the features. Additionally, when using monochromatic light to expose a photoresist layer, the light can pass through the layer and be reflected, thereby causing either constructive or destructive interference. A desired film thickness can be designed to operate at either a maxima or minima of the thin film interference/swing curve.
To produce small features in a uniform fashion, the uniformity of the photoresist layer is also important, meaning both the uniformity of the thickness of a photoresist film on a single substrate (the “intra-wafer uniformity”) and the uniformity of the (average) thickness between different coatings applied to different substrates (the “inter-wafer uniformity”). The intra-wafer uniformity is important, e.g., because it provides uniformity of the feature sizes of components placed on any given device. Inter-wafer uniformity is important, e.g., because producing coatings having predictably uniform thickness allows the production of devices having uniform and consistent quality.
As explained, the developed photoresist layer is a product of a multi-step process including coating a photoresist solution and coating a developer solution (after exposing the photoresist). Both of the process steps and their related materials can be key in producing a developed photoresist layer with uniform and predictable thicknesses, and with uniform feature sizes.
Spin processing methods attempt to provide coating uniformity by closely monitoring and/or controlling process conditions, materials, and individual process commands, to cause execution of spin-coating process steps in a uniform, repeatable fashion. This is generally accomplished by programming a computerized process control system to cause uniform execution of individual process steps with repetitive, predicted, timing and conditions, according to a pre-programmed set of events. Moreover, due to the very small dimensions and tolerances involved, factors surrounding the process that might otherwise be considered insignificant can have frustratingly real consequences in causing variability and non-uniformities of spin-coated materials. These can include the viscosity and temperature of the processing solution, spin speed and acceleration, process timing delays, air movement and velocity in the coating apparatus, ambient humidity, ambient temperature, ambient barometric pressure, chemical dispense system parameters, small variations in timing, mechanical impingement of applied processing solutions, etc. Certain methods exist to monitor and compensate for some of these factors to reduce their effects on the thickness of spin-coated materials.
Spin-coating processes typically account for and control processing conditions using a computerized process control system. One system often used for controlling spin-coating processes involves serial process control, e.g., a “round-robin”-type control process. In a serial-type control process, an electronic or computerized unit monitors and controls various elements of a spin-coating apparatus using a sequential or serial methodology. The process control system operates generally according to a continuous, serial (e.g., circular) path, sequentially addressing pre-identified components of the apparatus in a pre-determined order that does not vary (see FIG. 1). In practice, a computer or central processing unit (CPU) can be programmed to sequentially address one subroutine at a time. In FIG. 1, subroutines are represented by the rays emanating from the path followed by the CPU. The CPU addresses a subroutine, performs the instructions of the subroutine by checking conditions or parameters and taking any instructed action, and after any such action is taken, moving to the next subroutine.
Limits remain on the levels of coating uniformity attainable by spin-coating using methods of process control and methods of monitoring, controlling, or compensating for external conditions. This is especially true as feature sizes of microelectronic devices become smaller, and tolerances for variations in feature size become more demanding.